Modulator



June 10, 1952 R, M, FANO 2,599,905

MODULATOR Filed NOV. 29, 1945 MODULATED OUTPUT INVENTOR ROBERTO M4 FANO FIG.4 BY

ATTORNEY Patented une 1 0, 1952 UNITED {STATES PATENT OFFICE mesne 'assigmnents, to the United States of America asrepresented by the Secretary of the iNavy' imputation November 29, 1945, Serial No. 631,755

I 8 Claims. 1

This invention relates to apparatus for amplitude modulating radio frequency energy and more particularly to apparatus for amplitude modulating such energy by mechanical means.

It is sometimes necessary to generate a standard amplitude modulated signal whose percentage modulation is independent of frequency. The conventional way of varying the amplitude of a microwave signal is to introduce a piece of resistance card into the transmission line and periodically vary the amount of insertion. This proves to be unsatisfactory because the attenuation introduced by the card depends on the impedance of the load which in turn may vary with frequency.

It is an object of this invention to provide apparatus for amplitude modulating a microwave signal.

It is another object of this invention to provide apparatus for amplitude modulating a microwave signal by mechanical means.

It is a further object of this invention to provide apparatus for amplitude modulating a microwave signal such that the percentage modulation remains constant over a broad band of frequencies.

Other and further objects will appear during the course of the following description taken with the accompanying drawings in which:

Fig. 1 shows a sectional view of one embodiment of this invention;

Fig. 2 is a partially cut away and enlarged View of a portion of the embodiment of Fig. 1;

Fig. 3 is a sectional view along the line III-III in Fig. 2;

Fig. 4 is an equivalent circuit diagram inconstant voltage form of the embodiment of Fig. 1; and

Fig. 5 is an equivalent circuit diagram in constant current form of the embodiment of Fig. l.

The modulator consists, mainly, of a continuously variable coupler joining two waveguides (the word waveguide is here and'hereinafter used in a broad sense and includes any type of trans mission line). One guide is fed by an unmodulated source and is terminated into a load. Part of the power is coupled intothe second guide by means of the variable coupler. If the power coupled out of the first guideis small compared to the total power delivered by the source (1% or less), the amplitude of the signal in the second guide i very nearly independent of frequency. The coupling can be varied periodically to give the desired form and amount of amplitude 'modu lation.

The embodiment shown in-Fig. 1 comprises a continuously variable coupling between two ccaxial transmission lines It and l Asource of radio frequency energy 2 is connected a Polyiron matched load l3 by coaxial line H) Whose center conductor consists of a fixed section I (it and a section We which is 'rota'tablewith respect to outer conductor Illa by means of motor l5 through gears ti and 16b. The rotatable section We is flattened on two opposing sides in a taper fashion as shown in Fig. 2 and Fig. 3, and a rotating joint "effectively joins center conductors lflb and I00. This rotating joint com prises center conductor lilb rnilled out" to form a recessed pin l8 as shown. The adjacent end of center conductor I00 is shaped to receive a carbon bushing 19 which is adapted to enclose pin l8. Annular groove or channel 20 is thus. formed which, being one-half wavelength long, 'refiects the short circuit produced by terminating wall 2| at gap 22 between center conductors lb and We. Carbon bushing I9 serves as a bearing and also maintains proper alignment of center conductor Ills at therotat-ing joint while ball bearing 23 provides support at its other end. Joining coaxial line H) at right angles isanother coaxial line ll whose center conductor extends through slot 24 in outer conductor 0a and into coaxial line I0 to form probe H4. The position of probe [4 is adjustable by moving coaxial line H along coaxial line 1,0 in an axial direction.

Coaxial line 10 is fed by a'source of unmodulated radio frequency energy; matched load I 3 furnishing a non-critical terminationtc absorb the energy and prevent reflections over the necessary frequency range. Part of the energy is coupled into coaxial line by means of "ad-justable probe I4, the amount depending on the angular position of center conductor [9c for any given probe position. v j j The coupling can then be varied periodically by rotating the center conductor to give an amplitude modulated output having a waveform simi lar to a sine wave. The maximum coupling or the energy level is fixed substantially by the amount of insertion of probe IA while the-minimum coupling or the percentage modulation can be adjusted by moving probe IA along coaxial line ID in an axial direction. Thus the amount of amplitude modulation can be adjusted to the desired value.

Since the power coupled out by probe I4 is small compared to the'power dissipated interm at on 1.3, th m du a 'ono he output si nal is very nearly independent of the load impedance and the frequency. To demonstrate this, consider Fig. 4, the constant voltage equivalent circuit of the embodiment of Fig. l, in which Es is the voltage of source I2, 25 is the internal impedance of source l2, Z is the impedance of matched termination [3, Y is the admittance of the coupling between probe I4 and center conductor c, and Z1. is the impedance of the external load.

The probe coupling can be represented, to a first approximation, by the capacitance between probe [4 and center conductor Inc. Since the coupling impedance of the probe is much larger than both The modulation envelope, 1+f(t) depends only on the time variation of the coupling susceptance, provided, of course, the simplifying assumptions as to the magnitude of A V Y.

are satisfied. Moreover, if the susceptance Bo were controlled only by the capacitance Cc between probe l4 and center conductor I00, then and the modulation envelope would be independent of frequency. Although susceptance BC is controlled by other parameters besides Cc, the percentage modulation will in general remain substantially constant over a wide frequency band.

The modulation frequency is equal to twice the number of revolutions per second of center conductor I00 and can be adjusted to the desired value by varying the speed of driving motor It.

The term Polyiron indicates a material composed of powdered iron mixed with a phenolic binder.

This invention is only to be limited by the appended claims.

What is claimed is:

1. Apparatus for amplitude modulating radio frequency energy by means of a continuously variable coupling comprising, a coaxial transmission line having a rotatable center' conductor whose diameter varies along its axial direction,

said rotatable conductor being flattened to form two opposed sides a second coaxial transmission line coupled to said first coaxial line by means of an extension of the center conductor of said second coaxial line and forming the output connection, means for rotating said rotatable center conductor to vary the coupling between the two said coaxial lines, whereby the radio frequency energy in said first coaxial line is amplitude modulated as received by said second coaxial line.

2. Apparatus for amplitude modulating radio 4 frequency energy comprising, a coaxial transmission line having one end adapted for connection to a source of radio frequency energy, and having a load on the other end to absorb energy from said source, a portion of the inner conductor of said coaxial line being rotatable with respect to the remainder of said inner conductor, opposing sides of said rotatable portion being flattened in a taper fashion to provide recesses thereon, a second coaxial transmission line forming the output connection and coupled to said first coaxial line by means of an extension of the center conductor of said second coaxial line to form a probe adjacent said recesses on said rotatable center conductor, and means for rotating said rotatable center conductor to vary the coupling periodically between said probe and said rotatable center conductor, whereby the radio frequency energy from said source is amplitude modulated as received by said second coaxial line.

3. Apparatus for amplitude modulating the output of a microwave generator comprising, a coaxial transmission line having one end adapted for connection to, said generator and having a load on the other end to absorb energy from said generator, a portion of the inner conductor of said coaxial line adjacent said load being r0 tatable with respect to the remainder of said inner conductor, a rotating joint which efifectively joins the fixed portion of said inner conductor and said rotatable portion as well as providing an end bearing, opposing sides of said rotatable portion being flattened in a taper fashion to provide recesses thereon, a slot in the outer conductor of said first coaxial line, a second coaxial transmission line connected at right angles to said first coaxial line at said slot to form the output connection and being adjustable along said first coaxial line in an axial direction, an extension of the center conductor of said second coaxial line through said slot to form an adjustable probe adjacent said recesses on said rotatable center conductor, and means for rotating said rotatable center conductor to modulate the radio frequency energy amplitude from said generator as received in said second coaxial line.

4. Apparatus for amplitude modulating the radio frequency energy transmitted from one transmission line to. a second transmission line comprising, a variable coupling device having an outer cylindrical conductor and an inner rotatable center conductor, said inner rotatable center conductor being taperedalong its rotational axis and being formed to have opposed fiat surfaces, means to couple said first and second transmission lines to said device, and means to rotate said rotatable center conductor to vary the amount of coupling between said transmission lines.

5. Apparatus for amplitude modulating the radio frequency energy transmitted from one transmission line to a second transmission line comprising, a variable coupling device having an outer cylindrical conductor and an inner rotatable center conductor, said inner rotatable conductor being tapered along its rotational axis and being formed to have opposed fiat surfaces, means to couple said first and second transmission lines to said device, means to adjust the coupling of said second transmission line to said device, and means to rotate said rotatable center conductor to vary the amount of coupling periodically between said transmission lines.

6. Apparatus for amplitude modulating the radio frequency energy transmitted from one transmission line to a second transmission line through a coupling device comprising, a variable coupling device having an outer cylindrical conductor and an inner rotatable center conductor, said inner conductor having opposed fiat surfaces and being tapered along its rotational axis, means to couple said first and second transmission lines to said device, means to adjust the coupling to said second transmission line, means to rotate said rotatable center conductor to vary the coupling periodically between said transmission lines, and means to apply an electrical load to said inner rotatable conductor to absorb power from said first transmission line whereby the amplitude of energy in said second transmission line is substantially independent of frequency.

'7. Apparatus for amplitude modulating radio frequency energy by means of a continuously variable coupling comprising, a coaxial transmission line having one end adapted for connection to a source of radio frequency energy and having a load on the other end to absorb en- 6 coupling periodically between said probe and said rotatable center conductor.

8. Apparatus for amplitude modulating radio frequency energy bymeans of a continuously variable coupling comprising, a coaxial transmission having a rotatable center conductor Whose diameter varies along its axial direction, said rotatable conductor being flattened to form two opposed sides, a second coaxial transmission line coupled to said first coaxial line by means of an extension of the center conductor of said second coaxial line and forming the output connection, means to vary the coupling between said lines by adjusting the position of said second coaxial line with respect to said variable diameter conductor along the axial direction of said first coaxial line, and means for rotating said rotatable center conductor to vary the coupling periodically between the two said coaxial lines.

ROBERTO M. FAN 0.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,306,282 Samuel Dec. 22, 1942 2,342,254 Dallenbach Feb. 22, 1944 2,396,044 Fox Mar. 5, 1946 2,433,368 Johnson et a1 Dec. 30, 1947 2,451,825 Guarrera Oct. 19, 1948 2,483,818 Evans Oct. 4, 1949 2,514,678 Southworth July 11, 1950 

